1. Field of the Invention
The present invention relates generally to solid state lasers, and more particularly to a method and apparatus for controlling the pump distribution in a face pumped, diode pumped slab laser.
2. Description of the Related Art
A typical fluid-cooled slab laser includes an amplification medium in the form of a glass or crystalline solid slab, a pump such as a laser diode array which provides energy to the amplification medium to pump the atoms in the amplification medium to a higher energy level, and a cooling channel on one or more faces of the amplification medium to fluid cool the amplification medium. A laser beam is produced by oscillating a beam of coherent light through the amplification medium, whereby the beam of coherent light is amplified with each pass through the amplification medium.
In solid state lasers, a significant amount of heat is generated in the amplification medium by the energy from the pump. To maintain a high output power level in the laser, the heat can be dissipated by continuously flowing the cooling fluid through the cooling channel over the face of the slab. However, several optical distortions result from this arrangement. For example, a thermal gradient is produced within the slab, in which the inner portion of the slab is at a higher temperature than the outer portion. The thermal gradient causes a variation of the refractive index of the slab, known as "thermal lensing", which results in wavefront distortion of the coherent beam. In addition, fluid cooling of the slab causes mechanical stresses and strains which distort the major faces of the slab. The mechanical stresses and strains typically introduce a negative focal power at the edges of the slab, resulting in an additional wavefront distortion of the coherent beam. Fluid cooling also introduces variations in the refractive index of the slab due to a stress-optic effect.
The distortions of the slab are generally most pronounced in the edge regions of the slab. Thus, it is possible to avoid the optic effects of these distortions to a significant extent by confining the coherent beam to the central region of the slab. However, typical slabs (e.g. yttrium-aluminum-garnet) are limited in size by current crystal growth technology. Therefore, it is desirable to utilize the entire slab for amplification to optimize the utility of the laser.
Another known method of compensating for optical distortions involves propagating the coherent beam through different regions (e.g. inner and outer) of the slab by internal reflection so that each ray passes through substantially identical thermal environments. This method, which is described in commonly-owned U.S. Pat. No. 3,633,126 to Martin et al, does not, however, address the reduction of physical distortions of the slab.
Commonly-owned U.S. Pat. No. 4,730,324 to Azad discloses an apparatus for compensating for wavefront distortions in a slab laser which includes a pump lamp surrounded by a reflector which directs optical energy to the slab. The reflector has a shape which concentrates a selected quantity of optical radiation from the pump lamp into a central portion of the slab to introduce a positive focal power lens effect approximately equal in magnitude to the negative focal power lens effect at the lateral edges of the slab. The Azad apparatus, however, involves the formulation of a complex model of the laser configuration to determine the desired shape of the reflector based on ray tracing optimization, the pump cavity geometry, the optical properties of the components of the laser, and an emission model for the lamps which describes the wavelength and directional dependence of optical radiation emitted therefrom.
It would be desirable, therefore, to have a method and apparatus for selecting the pump distribution in a laser to reduce optical distortions without the need to formulate a complex optical model of all components of the laser. It would also be desirable to have a method and apparatus for adjusting the pump distribution in a laser while the laser is running to compensate for changing optical distortions on-line and to compensate for changes resulting from replacement of the slab.